Method for reinforcing rail by laser and auxiliary heat source efficient hybrid cladding

ABSTRACT

The disclosure discloses a method for reinforcing a rail by laser and auxiliary heat source efficient hybrid cladding. The laser and the auxiliary heat source simultaneously apply on a region to be cladded of a rail surface. The laser serves as a main heat source to enable simultaneous and rapid fusion of an added metal powder and partial substrate material in the rail surface to form a molten pool. The auxiliary heat source moves with the laser heat source in the same direction at the same speed, and performs synchronous preheating and/or post-heating on the laser molten pool, the heat-affected zone and the surface layer of the rail substrate to reduce the temperature gradient, thereby reducing the cooling rate, and avoiding martensite transformation and cracking in the heat-affected zone.

BACKGROUND Technical Field

The present disclosure belongs to the field of material processing, and particularly relates to a method for efficiently preparing a metal cladded coating on a rail surface by laser and auxiliary heat source hybrid cladding. The method can improve the wear resistance and contact fatigue performance of the rail, solve the problems of poor railway shunt, and repair the damaged rail.

Description of Related Art

China's rail transit has developed rapidly, and by the end of 2016, the national railway operation mileage has reached 124,000 kilometers. With the increase of railway transportation volume, train speed and axle load, damage problems such as rail wear, rolling contact fatigue and rail corrosion of rails are becoming more and more prominent. The damage of the rail mainly occurs on the surface, and thus, the preparation of the coating on the surface of the rail is of great significance for extending its service life.

Thermal spraying, electroplating and welding are the main methods currently used to prepare a metal coating on a rail surface. The thermal spray coating and the electroplating coating are mechanically bonded with the rail substrate, and are easy to fall off during the wheel-rail friction process due to the weak bonding force. The welding layer and the rail substrate are metallurgically bonded, but in this method, the heat input and heat-affected zone are large, resulting in that the microstructure and performance uniformity of the surfacing layer are poor, and the martensite structure is easily induced inside the rail substrate.

Compared with plasma arc and arc welding, laser cladding has the advantages of high energy density, small heat-affected zone, low heat input, low residual stress, small substrate penetration depth and high cladding efficiency, and is widely used in the preparation of surface strengthening coating and the additive manufacturing of metal parts. The Chinese Patent Application Publication No.: 107099793 discloses a method for improving the wear resistance of the wheel and rail in heavy-haul trains by using a laser-clad cobalt alloy coating, which uses a high-power laser to clad a cobalt alloy powder on the surface of the wheel and rail to reduce the surface friction coefficient thereof, thereby improving the wear resistance of the wheel rail and extending the service life of the wheel rail. However, with the rapid heating and rapid cooling effect of the laser, high-carbon acicular martensite structure may be generated in the heat-affected zone of the rail. The martensite structure has high hardness, but low toughness, which may easily cause the rail breakage. Therefore, the martensite structure in the rail shall be prohibited according to the railway industry standard TB/T2344-2003. In addition, since the cladded coating and the heat-affected zone have a high cooling rate and a large temperature gradient relative to the rail substrate at a high laser scanning rate, cracks may be easily generated in the cladded coating and the heat-affected zone, which affects the safe service of the train.

By combining a high-energy laser beam with an auxiliary heat source to perform hybrid processing, the above problems can be effectively solved. The Chinese Patent Publication No.: 101125394 discloses an automatic powder feeding laser induction hybrid cladding method and device, in which a laser and an induction heat source are used to perform hybrid processing, so that not only can the cladding efficiency be greatly improved, but also the problem that the alloy material with poor weldability are easily cracked during laser cladding can be solved. However, this method does not consider how to reduce and avoid the martensite phase transformation in the heat-affected zone when preparing a cladded coating on a large high-carbon steel substrate (such as a rail), as well as technical problems such as matching of mechanical properties of the cladded coating, the heat-affected zone and the substrate in a specific service environment (rolling rail contact).

SUMMARY

The present disclosure provides a method for efficiently preparing a high-performance cladded coating on a rail surface by laser and auxiliary heat source hybrid cladding to achieve the purpose of reinforcing and repairing the rail surface. A laser and an auxiliary heat source simultaneously apply on a region to be cladded of the rail surface, which not only avoids generation of cracks in the cladded coating and the heat-affected zone at a high laser scanning rate, but also avoids generation of harmful structures such as martensite in the heat-affected zone to ensure that mechanical properties between the cladded coating, the heat-affected zone and the rail substrate are well matched. The method of the present disclosure can be used for preparing a cladded coating on a rail surface to improve the wear resistance and contact fatigue performance of the rail, and can also solve the problems of poor railway shunt, repair of damaged rails and the like.

The present disclosure provides a method for reinforcing a rail by laser and auxiliary heat source efficient hybrid cladding, in which a laser and an auxiliary heat source are utilized to simultaneously apply on a region to be cladded of a rail surface; the laser serves as a main heat source to enable rapid fusion of a cladding material and a partial substrate material in the rail surface to form a molten pool; and the auxiliary heat source is located in front of or/and behind the laser heat source, moves with the laser heat source in the same direction at the same speed, and performs synchronous preheating and/or post-heating on the laser molten pool, a laser heat-affected zone and a surface layer of a rail substrate to reduce the cooling rate of the laser molten pool and heat-affected zone and avoid martensite transformation in the laser heat-affected zone and generation of cracks in the cladded coating and the heat-affected zone at a high laser scanning rate.

As an improvement of the above technical solution, a temperature cycle curve of the heat-affected zone under laser action is reasonably regulated by the combined action of the laser and the auxiliary heat source such that a cooling time of the heat-affected zone is larger than a critical cooling time of transformation from austenite to pearlite in a continuous cooling transformation curve (CCT curve) or a time-temperature-transformation curve (TTT curve), thereby meeting critical conditions of complete transformation from austenite to pearlite, and allowing the heat-affected zone to be transformed into a fine lamellar pearlite structure which has an interlamellar spacing less than or equal to that of the rail substrate and has a hardness between those of the cladded coating and the rail substrate, so that mechanical properties between the cladded coating, the heat-affected zone and the rail substrate are reasonably matched, the hardness curve is smooth, and the overall fatigue performance is good.

As a further improvement of the above technical solution, the auxiliary heat source adopts any one of induction heating, oxyacetylene flame and propane torch, or any combination thereof; the preheating temperature is 100-1000° C., and the post-heating temperature is 300-700° C.; a metal cladded coating obtained by single processing has a thickness of 0.1-2 mm, a width of 3-20 mm, and a hardness which is adjustable within a range of HV250 to HV500 according to specific requirements of the rail; the laser heat-affected zone has a width of less than 1 mm and a hardness of HV250 to HV400, which can avoid martensite transformation in the heat-affected zone; and the induction heating is performed by an induction power supply and an induction coil. The induction coil is manufactured by bending and welding a copper tube, magnets are embedded on the copper tube in a working area, a lower surface of the copper tube is parallel to a cladding surface of the rail, with a gap of 0.5-15 mm; and a heating surface has a linear structure along a longitudinal direction of the rail, and has a length of 10-500 mm.

The method according to the present disclosure comprises following specific implementation steps.

(1) Polish the region to be cladded of the rail surface first to remove surface rust and contaminants.

(2) Adjust a defocusing distance of a laser beam to allow a laser spot to be a circular spot with a diameter of 3-20 mm or a rectangular spot with a size of (1-3) mm×(6-30) mm.

(3) Adjust relative position of the laser spot and the auxiliary heat source such that the laser spot is in front of, in the middle of or behind the auxiliary heat source.

(4) Turn on the laser and the auxiliary heat source, and synchronously feed or pre-place a coating material into a laser irradiation region of the rail surface by using an automatic powder feeder, so that the molten pool is formed when the focused laser beam is incident on the rail substrate, and then a metal cladded coating is formed on the rail surface after the molten pool is solidified, wherein the auxiliary heat source plays a role of preheating and/or post-heating the rail, with a preheating temperature of 100-1000° C. and a post-heating temperature of 300-700° C.

(5) After a layer of the metal cladded coating is formed, determine whether a thickness of the cladded coating meets working conditions, and if so, end the cladding process; if not, repeat the above steps (2), (3) and (4) until the thickness requirements are met.

(6) After the cladding process is finished, inspect the surface of the corrosion-resistant cladded coating by penetration or ultrasonic inspection, to ensure that there are no metallurgical defects in the cladded coating.

(7) Selectively perform cleaning and profile trimming on a rail tread to make its surface flat.

The cladded coating material may be an iron-based alloy, main chemical compositions (by weight percentage) of which are: (0.01-0.60) C, (10-40) Cr, (5-18) Ni, (0.1-3.0) Si, (0-3) B, (0-3) Mo, (1-3) Mn and Fe balance.

The cladded coating material may be a nickel-based alloy, main chemical compositions (by weight percentage) of which are: (0.01-0.50) C, (20-30) Cr, (5-10) W, (3-5) Si, (0-3) B, (5-10) Fe and Ni balance.

The cladded coating material may be a cobalt-based alloy, main chemical compositions (by weight percentage) of which are: (0.01-0.5) C, (20-35) Cr, (1-10) Ni, (1-3) Si, (5-15) W, (0-3) B, (0.5-2) Mn and Co balance.

The present disclosure has the following beneficial effects:

{circle around (1)} The laser and the auxiliary heat source simultaneously apply on a region to be cladded of a rail surface; the high-energy laser beam enables rapid fusion of a cladded coating material and a thin-layer material on the rail surface to form a molten pool; and the auxiliary heat source performs synchronous preheating and post-heating on the laser molten pool, a heat-affected zone and a surface layer of the rail substrate to reduce a temperature gradient between the laser molten pool, the heat-affected zone and the rail substrate, thereby reducing the cooling rate, and avoiding cracking and spalling of the metal cladded coating and the heat-affected zone at a high laser scanning rate.

{circle around (2)} Through adjusting the relative position of the laser and the auxiliary heat source, the laser processing power, the laser scanning rate, and the heating temperature of the auxiliary heat source to the rail, a temperature cycle curve of the heat-affected zone can be reasonably controlled such that a cooling time of the heat-affected zone is larger than a critical cooling time of transformation from austenite to pearlite in the CCT curve or the TTT curve, thereby meeting critical conditions of complete transformation from austenite to pearlite, and allowing the heat-affected zone to be transformed into a fine lamellar pearlite structure which has an interlamellar spacing less than or equal to that of the rail substrate and has a hardness between those of the cladded coating and the rail substrate, so that mechanical properties between the cladded coating, the heat-affected zone and the rail substrate are reasonably matched, the hardness curve is smooth, and the overall fatigue performance is good.

{circle around (3)} Compared with other methods such as plasma arc and electric arc, the laser energy density is high, the heat-affected zone is small, the martensite structure in the heat-affected zone can be eliminate while the heating width and depth of the induction coil and other auxiliary heat sources are just larger than the heat-affected zone width, so that the overall heat input is small resulting in small processing residual stress and deformation, and high stability of the rail; the device has good flexibility and high processing precision, and can repair rails with different degrees of damage.

{circle around (4)} The cladded coating has low dilution rate, especially for a thinner cladded coating. When the thickness of the cladded coating is less than 0.5 mm, the dilution rate of the coating is less than 5%, which can ensure the wear resistance and corrosion resistance of the cladded coating.

{circle around (5)} Due to the combination of the auxiliary heat source, the energy required to form the molten pool is greatly reduced. When the laser power is 1-20 kW, the deposition rate of the cladded coating can reach 10-250 g/min, and the scanning speed reaches 0.4-30 m/min. Compared with the pure laser cladding process, the deposition efficiency is increased by 3-15 times.

The present disclosure has strong versatility, and can efficiently prepare a wear-resistant, corrosion-resistant and fatigue-resistant cladded coating with uniform composition and matched mechanical properties directly on the rail surface, and can also repair a locally damaged rail. The hardness distribution curve of the reinforced or repaired rail along the depth direction is smooth, the mechanical properties of the cladded coating, the heat-affected zone and the substrate are matched with each other, and the overall fatigue performance is good, so that the coating spalling does not occur during the service. Meanwhile, various process components used in the method in the present disclosure have high integration degree and are easy to integrate with related processing platforms, and the method can be used for both a fixed laser processing machine to perform off-line processing and mobile laser processing equipment (e.g., a mobile laser processing vehicle) to perform on-line reinforcement or repair at the railway site.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view showing laser and induction post-heating hybrid cladding on a rail surface in a case where the laser spot is located in front of induction heating.

FIG. 2 is a top view showing laser, induction preheating and induction post-heating hybrid cladding on a rail surface in a case where the laser spot is located in the middle of induction heating.

FIG. 3 is a top view showing laser, induction heating and oxyacetylene flame (or laser, induction heating and propane torch) heating hybrid cladding on a rail surface.

FIG. 4 is a top view showing laser and oxyacetylene flame (or laser and propane torch) heating hybrid cladding on a rail surface.

FIG. 5 is a hardness distribution diagram along the depth direction of a rail with a

Fe-based metal cladded coating prepared by laser and induction post-heating hybrid cladding.

FIG. 6 is a hardness distribution diagram along the depth direction of a rail with a Ni-based metal cladded coating prepared by laser, induction preheating and induction post-heating hybrid cladding.

FIG. 7 is a hardness distribution diagram along the depth direction of a rail with a Co-based metal cladded coating prepared by laser, induction heating and oxyacetylene flame (or laser, induction heating and propane torch) heating hybrid cladding.

FIG. 8 is a hardness distribution diagram along the depth direction of a rail with a Fe-based metal cladded coating prepared by laser and oxyacetylene flame (or laser and propane torch) heating hybrid cladding.

In the figures: 1, laser spot; 2, induction heating coil; 2(a), induction preheating coil; 2(b), induction post-heating coil; and 3, oxyacetylene flame (or propane torch).

DESCRIPTION OF THE EMBODIMENTS

For clear understanding of the objectives, features and advantages of the present disclosure, detailed description of the present disclosure will be given below in conjunction with accompanying drawings and specific embodiments. It should be noted that the embodiments described herein are only meant to explain the present disclosure, and not to limit the scope of the present disclosure. Furthermore, the technical features related to the embodiments of the disclosure described below can be mutually combined if they are not found to be mutually exclusive.

In the present disclosure, a laser is used as a main heat source to deposit an alloy material on a surface of a rail, and an auxiliary heat source preheats or/and post-heats the rail to reduce the cooling rate of the cladded coating and the heat-affected zone, so that a functional coating with wear resistance, fatigue resistance and corrosion resistance can be efficiently prepared on the rail surface, or the damaged rail can be repaired. The thickness of the cladded coating obtained in a single processing is 0.1-2 mm, and the hardness can be adjusted in a range of HV250 to HV500 according to the specific requirements of the rail. Meanwhile, by adopting the technical route proposed by the present disclosure, martensite is not generated in the heat-affected zone of the rail, and the mechanical properties of the cladded coating and the rail substrate are reasonably matched, so that the rail has better bending fatigue and contact fatigue performance while being reinforced and repaired. The present disclosure will be further described below with reference to the accompanying drawings and embodiments.

The present disclosure provides a method for reinforcing a rail by using laser and auxiliary heat source hybrid cladding, in which the auxiliary heat source may adopt induction heating, oxyacetylene flame, propane torch or a combination of induction heating and oxyacetylene flame (or propane torch). The present method can be integrated with fixed laser processing equipment to perform off-line processing on a rail, or integrated with a vehicle-mounted laser processing platform to perform on-line reinforcement or repair on a rail at the railway site. The implementation steps include the following.

(1) Polish a region to be cladded of a rail surface to remove surface rust and contaminants.

(2) Adjust the defocusing distance of a laser beam to allow a laser spot to be circular with a diameter of 3-20 mm or rectangular with a size of (1-3) mm×(6-30) mm.

(3) Adjust the relative position of the laser spot and the auxiliary heat source so that the laser spot is in front of, in the middle of or behind the auxiliary heat source.

(4) Turn on the laser and auxiliary heat source, and synchronously feed (or place in advance) the alloy powder material into the laser irradiation region of the rail surface by using an automatic powder feeder, so that a molten pool is formed when the focused laser beam irradiates on the rail substrate, and then a metal coating is formed on the rail surface after the molten pool is solidified; the auxiliary heat source acts on the rail for preheating and/or post-heating, with a preheating temperature of 100-1000° C. and a post-heating temperature of 300-700° C.

(5) After a layer of metal cladded coating is formed, determine whether the thickness of the cladded coating meets working conditions, and if so, end the cladding process; if not, repeat the above steps (2), (3) and (4) until the thickness requirements are met.

(6) After the cladding process is finished, inspect the surface of the corrosion-resistant cladded coating by penetration or ultrasonic inspection, to ensure that there are no metallurgical defects in the cladded coating.

(7) According to application requirements, selectively perform cleaning and profile trimming on the rail tread to make the surface flat, thereby obtaining a finished product.

Embodiment 1: on-line laser and induction post-heating efficient hybrid cladding at the railway site

In this embodiment, the service rail is efficiently reinforced or repaired at the railway site, in which induction heating acts as an auxiliary heat source, and an industrial manipulator or a three-dimensional motion axis is used as a machining motion and position control unit. A region to be cladded of a rail surface is heated with the heating temperature and time controlled by an induction heating device and a temperature control part. The induction heating device includes an induction power supply and an induction coil, and the temperature control part includes an infrared thermometer and a temperature controller, in which the induction coil is connected to the induction power supply, the infrared thermometer is connected to the temperature controller, and the temperature controller is connected to the induction power supply via a data line. A detection signal of the infrared thermometer is input to the temperature controller, and after calculation, the temperature controller outputs a control signal to adjust the output power of the induction heating power supply to achieve the control of the induction heating temperature of the rail. The laser spot is focused on the front of the induction coil, as shown in FIG. 1. Basic implementation steps for preparing a metal cladded coating on a rail surface by laser and induction post-heating hybrid cladding are as follows.

(1) Select iron-based alloy powder as cladding material, the main chemical compositions (Wt. %) of which are: (0.01-0.60) C, (10-40) Cr, (5-18) Ni, (0.1-3.0) Si, (0-3) B, (0-3) Mo, (1-3) Mn and Fe balance.

(2) Polish a region to be cladded of a rail surface to remove surface rust and contaminants.

(3) Adjust the position of the induction coil such that its lower surface is parallel to the region to be cladded of the rail surface with a gap of 5 mm; the infrared thermometer is targeted at the induction heating region of the rail surface, the infrared thermometer is connected to the temperature controller and the induction power supply to detect and control the induction heating temperature; the induction heating temperature is set to 700° C.

(4) Adjust the defocusing distance of a laser beam and the relative position of the laser spot and the induction coil such that the laser spot is focused on a rail surface in front of the induction coil; the laser spot is a circular spot with a diameter of 3 mm, the powder feed rate of a powder feeder is 10 g/min, the laser power is 1 kW, and the laser scanning speed is 0.4 m/min.

(5) Turn on a motion control unit, the laser and the induction heating power supply, and synchronously feed (or pre-place) the cladding material into the laser irradiation region of the rail surface by using an automatic powder feeder, so that a molten pool is formed when the focused laser beam is incident on the rail substrate, and then a metal cladded coating is formed on the rail surface after the molten pool is solidified.

(6) After a layer of cladded coating is formed, determine whether the thickness of the cladded coating meets working conditions, and if so, end the cladding process; if not, repeat the above steps (2), (3), (4) and (5) until the thickness requirements are met.

(7) After the cladding process is finished, inspect the surface of the metal coating by penetration or ultrasonic inspection, to ensure that there are no metallurgical defects in the cladded coating.

(8) According to application requirements, selectively perform cleaning and profile trimming on the rail tread to make the surface flat, thereby obtaining a finished product.

In this embodiment, the thickness of the prepared iron-based metal cladded coating is 0.1 mm, the mechanical properties between the cladded coating, the heat-affected zone and the rail substrate are reasonably matched, and the hardness distribution along the rail depth direction is as shown in FIG. 5.

Embodiment 2: on-line laser, induction preheating and induction post-heating efficient hybrid cladding at the railway site

In this embodiment, the service rail is efficiently reinforced or repaired at the railway site, in which induction heating acts as an auxiliary heat source, the induction heating control part is the same as that in Embodiment 1, and an industrial manipulator or a three-dimensional motion axis is used as a machining motion and position control unit. The laser spot is focused in the middle of the induction coil, as shown in FIG. 2. Basic implementation steps for preparing a metal cladded coating on a rail surface by laser, induction preheating and induction post-heating hybrid cladding are as follows.

(1) Select nickel-based alloy powder as cladding material, the main chemical compositions (Wt. %) of which are: (0.01-0.50) C, (20-30) Cr, (5-10) W, (3-5) Si, (0-3) B, (5-10) Fe and Ni balance.

(2) Polish a region to be cladded of a rail surface to remove surface rust and contaminants.

(3) Adjust the position of the induction coil such that the lower surface is parallel to the region to be cladded of the rail surface with a gap of 0.5 mm; the infrared thermometer is targeted at the induction heating region of the rail surface, and the infrared thermometer is connected to the temperature controller and the induction power supply to detect and control the induction heating temperature; the induction heating temperature is set to 500° C.

(4) Adjust the defocusing distance of a laser beam and the relative position of the laser spot and the induction coil such that the laser spot is focused on a rail surface in front of the induction coil; the laser spot is a rectangular spot with a size of 1×6 mm, the powder feed rate of a powder feeder is 50 g/min, the laser power is 5 kW, and the laser scanning speed is 2 m/min.

(5) Turn on a motion control unit, the laser and the induction heating power supply, and synchronously feed (or pre-place) the cladding material into the laser irradiation region of the rail surface by using an automatic powder feeder, so that a molten pool is formed when the focused laser beam is incident on the rail substrate, and then a metal cladded coating is formed on the rail surface after the molten pool is solidified.

(6) After a layer of cladded coating is formed, determine whether the thickness of the cladded coating meets working conditions, and if so, end the cladding process; if not, repeat the above steps (2), (3), (4) and (5) until the thickness requirements are met.

(7) After the cladding process is finished, inspect the surface of the metal coating by penetration or ultrasonic inspection, to ensure that there are no metallurgical defects in the cladded coating.

(8) According to application requirements, selectively perform cleaning and profile trimming on the rail tread to make the surface flat, thereby obtaining a finished product.

In this embodiment, the induction coil consists of two parts: 4(a) and 4(b) connected by a copper tube, in which 4(a) plays a role of preheating the rail, and 4(b) plays a role of delaying the cooling rate of the rail. In practical applications, under the premise of reasonable matching of mechanical properties, the cladding efficiency can be effectively improved, which is conducive to energy conservation. The thickness of the prepared nickel-based metal cladded coating is 0.5 mm, the mechanical properties between the cladded coating, the heat-affected zone and the rail substrate are reasonably matched, and the hardness distribution along the rail depth direction is as shown in FIG. 6.

Embodiment 3: off-line laser, induction heating and oxyacetylene flame (or propane torch) heating efficient hybrid cladding on a rail surface

In this embodiment, off-line reinforcement or repair is performed on the rail, in which induction heating and oxyacetylene flame (or propane torch) act as the auxiliary heat source. The laser spot is focused in front of the induction coil, and the oxyacetylene flame (or propane torch) preheats a rail surface to be cladded, as shown in FIG. 3. The laser and the induction coil move in the same direction at the same speed, and the induction heating synchronously preheats the laser molten pool and the heat-affected zone of the rail. The basic implementation steps are as follows:

(1) Select cobalt-based alloy powder as cladding material, the main chemical compositions (Wt. %) of which are: (0.01-0.5) C, (20-35) Cr, (1-10) Ni, (1-3) Si, (5-15) W, (0-3) B, (0.5-2) Mn and Co balance.

(2) Polish a region to be cladded of a rail surface to remove surface rust and contaminants.

(3) Adjust the position of the induction coil such that the lower surface is parallel to the region to be cladded of the rail surface with a gap of 15 mm; the infrared thermometer is targeted at the induction heating region of the rail surface, and the infrared thermometer is connected to the temperature controller and the induction power supply to detect and control the induction heating temperature; the induction heating temperature is set to 300° C.

(4) Adjust the defocusing distance of a laser beam and the relative position of the laser spot and the induction coil so that the laser spot is focused on a rail surface in front of the induction coil; the laser spot is a rectangular spot with a size of 3×30 mm, the powder feed rate of a powder feeder is 250 g/min, the laser power is 20 kW, and the laser scanning speed is 30 m/min.

(5) Preheat the region to be cladded of the rail surface with oxyacetylene flame/propane torch, in which the infrared thermometer 6-2 is aimed at the heated region of the rail surface to monitor the preheating temperature, and when the preheating temperature reaches 100-200° C., the oxyacetylene flame/propane torch device is closed.

(6) Turn on the laser and the induction heating power supply, and synchronously feed (or pre-place) the cladding material into the laser irradiation region of the rail surface by using an automatic powder feeder, so that a molten pool is formed when the focused laser beam is incident on the rail substrate, and then a metal cladded coating is formed on the rail surface after the molten pool is solidified.

(7) After a layer of cladded coating is formed, determine whether the thickness of the cladded coating meets working conditions, and if so, end the cladding process; if not, repeat the above steps (2), (3), (4), (5) and (6) until the thickness requirements are met.

(8) After the cladding process is finished, inspect the surface of the metal coating by penetration or ultrasonic inspection, to ensure that there are no metallurgical defects in the cladded coating.

(9) According to application requirements, selectively perform cleaning and profile trimming on the rail tread to make the surface flat, thereby obtaining a finished product.

In this embodiment, the thickness of the prepared cobalt-based metal cladded coating is 0.2 mm, the mechanical properties between the cladded coating, the heat-affected zone and the rail substrate are reasonably matched, and the hardness distribution along the rail depth direction is as shown in FIG. 7.

Embodiment 4: off-line laser and oxyacetylene flame (or propane torch) heating efficient hybrid cladding on a rail surface

In this embodiment, off-line reinforcement and repair is performed on the rail, in which oxyacetylene flame (or propane torch) is selected as an auxiliary heat source. As shown in FIG. 4, basic implementation steps for preparing a metal cladded coating on a rail surface by laser and oxyacetylene flame (or laser and propane torch) hybrid cladding are as follows.

(1) Select iron-based alloy powder as cladding material, the main chemical compositions (Wt. %) of which are: (0.01-0.60) C, (10-40) Cr, (5-18) Ni, (0.1-3.0) Si, (0-3) B, (0-3) Mo, (1-3) Mn and Fe balance.

(2) Polish a region to be cladded of a rail surface to remove surface rust and contaminants.

(3) adjust parameters such that the laser beam is circular with a diameter of 20 mm, the laser power is 15 kW, the powder feed rate of a powder feeder is 180 g/min, and the laser scanning speed is 10 m/min.

(4) Preheat the region to be cladded of the rail surface with oxyacetylene flame/propane torch, in which the infrared thermometer is aimed at the heated region of the rail surface and monitors the preheating temperature of the rail surface to be 800-1000° C.

(5) Turn on the laser, and synchronously feed (or pre-place) the alloy powder material into the laser irradiation region of the rail surface by using an automatic powder feeder, so that a molten pool is formed when the focused laser beam is incident on the rail substrate, and then a metal cladded coating is formed on the rail surface after the molten pool is solidified; meanwhile, perform post-heating on the cladded region of the rail surface with the oxyacetylene flame/propane torch at a post-heating temperature of 300-400° C. (monitored by the infrared thermometer), and turn off the oxyacetylene flame/propane torch after a certain holding time.

(6) After a layer of cladded coating is formed, determine whether the thickness of the cladded coating meets working conditions, and if so, end the cladding process; if not, repeat the above steps (3), (4) and (5) until the thickness requirements are met.

(7) After the cladding process is finished, inspect the surface of the metal coating by penetration or ultrasonic inspection, to ensure that there are no metallurgical defects in the cladded coating.

(8) According to application requirements, selectively perform cleaning and profile trimming on the rail tread to make the surface flat, thereby obtaining a finished product.

In this embodiment, the thickness of the prepared iron-based metal cladded coating is 2 mm, the mechanical properties between the cladded coating, the heat-affected zone and the rail substrate are reasonably matched, and the hardness distribution along the rail depth direction is as shown in FIG. 8.

It should be readily understood to those skilled in the art that the above description is only preferred embodiments of the present disclosure, and does not limit the scope of the present disclosure. Any change, equivalent substitution and modification made without departing from the spirit and scope of the present disclosure should be included within the scope of the protection of the present disclosure. 

1. A method for reinforcing a rail by laser and auxiliary heat source efficient hybrid cladding, wherein in the method, a laser and an auxiliary heat source are utilized to simultaneously apply on a region to be cladded of a rail surface; the laser serves as a main heat source to enable rapid fusion of an added powder material and a partial substrate material on the rail surface to form a molten pool and then to form a cladded coating; the auxiliary heat source is located in front of or/and behind the main heat source, moves with the main heat source in the same direction at the same speed, and performs synchronous preheating and/or post-heating on the molten pool, a heat-affected zone and a surface layer of a rail substrate to reduce a temperature gradient between the molten pool and heat-affected zone and the rail substrate, thereby reducing a cooling rate of the molten pool and heat-affected zone, and avoiding martensite transformation in the laser-heat-affected zone and generation of cracks in the cladded coating and the heat-affected zone at a high laser scanning rate.
 2. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 1, wherein a thermal cycle process of the heat-affected zone under laser action is reasonably regulated by the combined action of the laser and the auxiliary heat source such that a cooling time of the heat-affected zone is larger than a critical cooling time of transformation from austenite to pearlite in a continuous cooling transformation (CCT) curve or a time-temperature-transformation (TTT) curve, thereby meeting critical conditions of complete transformation from austenite to pearlite, and allowing the heat-affected zone to be transformed into a fine lamellar pearlite structure which has an interlamellar spacing less than or equal to that of the rail substrate and has a hardness between hardnesses of the cladded coating and the rail substrate, so that mechanical properties between the cladded coating, the heat-affected zone and the rail substrate are reasonably matched, the hardness curve is smooth, and the overall fatigue performance is good.
 3. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 1, wherein the auxiliary heat source adopts any one of induction heating, oxyacetylene flame and propane torch, or any combination thereof.
 4. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 1, wherein a preheating temperature is 100-1000° C., and a post-heating temperature is 300-700° C.
 5. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 1, wherein the cladded coating obtained by single processing has a thickness of 0.1-2 mm, a width of 3-20 mm, and a hardness which is controlled within a range of HV250 to HV500 according to specific requirements of the rail.
 6. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 1, wherein the heat-affected zone has a width of less than 1 mm and a hardness of HV250 to HV400, and there is no martensite transformation in the heat-affected zone.
 7. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 1, wherein the method comprises following specific implementation steps of: (1) polishing the region to be cladded of the rail surface first to remove surface rust and contaminants; (2) adjusting a defocusing distance of a laser beam to allow a laser spot to be a circular spot with a diameter of 3-20 mm or a rectangular spot with a size of (1-3) mm×(6-30) mm; (3) adjusting relative position of the laser spot and the auxiliary heat source such that the laser spot is in front of, in the middle of or behind the auxiliary heat source; (4) turning on the laser and the auxiliary heat source, and synchronously feeding or pre-placing a coating material into a laser irradiation region of the rail surface by using an automatic powder feeder, so that the molten pool is formed when the focused laser beam is incident on the rail substrate, and then the cladded coating is formed on the rail surface after the molten pool is solidified, wherein the auxiliary heat source plays a role of preheating and/or post-heating the rail, with a preheating temperature of 100-1000° C. and a post-heating temperature of 300-700° C.; (5) after a layer of the cladded coating is formed, determining whether a thickness of the cladded coating meets working conditions, and if so, ending the cladding process; if not, repeating the above steps (2), (3) and (4) until the thickness requirements are met; (6) after the cladding process is finished, inspecting the surface of the corrosion-resistant cladded coating by penetration or ultrasonic inspection, to ensure that there are no metallurgical defects in the cladded coating; and (7) selectively performing cleaning and profile trimming on a rail tread to make its surface flat.
 8. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 1, wherein the method is integrated with a fixed processing platform to perform off-line processing of the rail, or integrated with an on-line mobile laser processing vehicle to perform on-line laser cladding reinforcement or repair of the rail at a railway site.
 9. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 3, wherein the induction heating is implemented by an induction power supply and an induction coil; wherein the induction coil is formed by bending and welding a copper tube, a magnet is embedded on the copper tube in a working area, a lower surface of the copper tube is parallel to a cladded surface of the rail, with a gap of 0.5-15 mm; a heating zone on the rail surface has a linear structure, which is parallel to a longitudinal direction of the rail and has a length of 10-500 mm.
 10. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 1, wherein the added powder material is an iron-based alloy, main chemical compositions of which are: 0.01-0.60% C, 10-40% Cr, 5-18% Ni, 0.1-3.0% Si, 0-3% B, 0-3% Mo, 1-3% Mn and Fe balance; or the added powder material is a nickel-based alloy or a cobalt-based alloy, wherein main chemical compositions of the nickel-based alloy are: 0.01-0.50% C, 20-30% Cr, 5-10% W, 3-5% Si, 0-3% B, 5-10% Fe and Ni balance; and main chemical compositions of the cobalt-based alloy are: 0.01-0.5% C, 20-35% Cr, 1-10% Ni, 1-3% Si, 5-15% W, 0-3% B, 0.5-2% Mn and Co balance.
 11. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 2, wherein the auxiliary heat source adopts any one of induction heating, oxyacetylene flame and propane torch, or any combination thereof.
 12. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 2, wherein a preheating temperature is 100-1000° C., and a post-heating temperature is 300-700° C.
 13. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 2, wherein the cladded coating obtained by single processing has a thickness of 0.1-2 mm, a width of 3-20 mm, and a hardness which is controlled within a range of HV250 to HV500 according to specific requirements of the rail.
 14. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 2, wherein the heat-affected zone has a width of less than 1 mm and a hardness of HV250 to HV400, and there is no martensite transformation in the heat-affected zone.
 15. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 2, wherein the method comprises following specific implementation steps of: (1) polishing the region to be cladded of the rail surface first to remove surface rust and contaminants; (2) adjusting a defocusing distance of a laser beam to allow a laser spot to be a circular spot with a diameter of 3-20 mm or a rectangular spot with a size of (1-3) mm×(6-30) mm; (3) adjusting relative position of the laser spot and the auxiliary heat source such that the laser spot is in front of, in the middle of or behind the auxiliary heat source; (4) turning on the laser and the auxiliary heat source, and synchronously feeding or pre-placing a coating material into a laser irradiation region of the rail surface by using an automatic powder feeder, so that the molten pool is formed when the focused laser beam is incident on the rail substrate, and then the cladded coating is formed on the rail surface after the molten pool is solidified, wherein the auxiliary heat source plays a role of preheating and/or post-heating the rail, with a preheating temperature of 100-1000° C. and a post-heating temperature of 300-700° C.; (5) after a layer of the cladded coating is formed, determining whether a thickness of the cladded coating meets working conditions, and if so, ending the cladding process; if not, repeating the above steps (2), (3) and (4) until the thickness requirements are met; (6) after the cladding process is finished, inspecting the surface of the corrosion-resistant cladded coating by penetration or ultrasonic inspection, to ensure that there are no metallurgical defects in the cladded coating; and (7) selectively performing cleaning and profile trimming on a rail tread to make its surface flat.
 16. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 2, wherein the method is integrated with a fixed processing platform to perform off-line processing of the rail, or integrated with an on-line mobile laser processing vehicle to perform on-line laser cladding reinforcement or repair of the rail at a railway site.
 17. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 11, wherein the induction heating is implemented by an induction power supply and an induction coil; wherein the induction coil is formed by bending and welding a copper tube, a magnet is embedded on the copper tube in a working area, a lower surface of the copper tube is parallel to a cladded surface of the rail, with a gap of 0.5-15 mm; a heating zone on the rail surface has a linear structure, which is parallel to a longitudinal direction of the rail and has a length of 10-500 mm.
 18. The method for reinforcing the rail by laser and auxiliary heat source efficient hybrid cladding according to claim 2, wherein the added powder material is an iron-based alloy, main chemical compositions (by weight percentage) of which are: 0.01-0.60% C, 10-40% Cr, 5-18% Ni, 0.1-3.0% Si, 0-3% B, 0-3% Mo, 1-3% Mn and Fe balanc; or the added powder material is a nickel-based alloy or a cobalt-based alloy, wherein main chemical compositions (by weight percentage) of the nickel-based alloy are: 0.01-0.50% C, 20-30% Cr, 5-10% W, 3-5% Si, 0-3% B, 5-10% Fe and Ni balance; and main chemical compositions (by weight percentage) of the cobalt-based alloy are: 0.01-0.5% C, 20-35% Cr, 1-310% Ni, 1-3% Si, 5-15% W, 0-3% B, 0.5-2% Mn and Co balance. 